Unidirectionally-solidification process and castings formed thereby

ABSTRACT

A process capable of producing large metallic castings having lengths of one hundred centimeters or more and a unidirectional crystal structure substantially free of freckle defects. The process includes pouring a molten metal alloy into a preheated mold within a heating zone, withdrawing the mold from the heating zone, through a heat shield, and into a cooling zone to directionally solidify the molten metal alloy, and then cooling the mold to produce the casting and the unidirectional crystal structure thereof. The heat shield operates as a barrier to thermal radiation between the heating zone and the cooling zone, and the mold is withdrawn at a rate that, in combination with the heat shield, maintains a thermal gradient to solidify the molten metal alloy and form primary dendrite arms having an average spacing therebetween of about 150 to about 500 micrometers.

BACKGROUND OF THE INVENTION

The present invention generally relates to materials and processes forproducing directionally-solidified castings, and particularly toreducing defects in alloys cast as single-crystal (SX) anddirectionally-solidified (DS) articles suitable for use as components ofgas turbines and other high temperature applications.

Components of gas turbines, such as blades (buckets), vanes (nozzles)and combustor components, are typically formed of nickel, cobalt oriron-base superalloys characterized by desirable mechanical propertiesat turbine operating temperatures. Because the efficiency of a gasturbine is dependent on its operating temperatures, there is a demandfor components, and particularly turbine buckets, nozzles, combustorcomponents, and other hot gas path components, that are capable ofwithstanding higher temperatures. As the material requirements for gasturbine components have increased, various processing methods andalloying constituents have been used to enhance the mechanical, physicaland environmental properties of components formed from superalloys. Forexample, buckets, nozzles and other components employed in demandingapplications are often cast by directional casting techniques to have DSor SX microstructures, characterized by a crystal orientation or growthdirection in a selected direction to produce columnar polycrystalline orsingle-crystal articles.

As known in the art, directional casting techniques for producing SX andDS castings generally entail pouring a melt of the desired alloy into aninvestment mold held at a temperature above the liquidus temperature ofthe alloy. One such process employs a Bridgman-type furnace to create aheated zone surrounding the mold, and a chill plate at the base of themold. Solidification of the molten alloy within the mold occurs bygradually withdrawing the mold from the heated zone and into a coolingzone, where cooling occurs by convection and/or radiation.Solidification initiates at the base of the mold and the solidificationfront progresses to the top of the mold. Solidification is initiated andcontrolled within the mold base in a manner that obtains the desiredmicrostructure for the casting. A high thermal gradient is required atthe solidification front to prevent nucleation of new grains duringdirectional solidification processes.

As known in the art, dendrites are tree-like structures that form duringthe solidification of a molten metal. The spacing between dendrite armsin a casting is influenced by the solidification conditions of thecasting, with dendrite arm spacing varying inversely with cooling rate.As used herein, primary dendrite arm spacing will be used to denote theaverage spacing between cores of adjacent dendrites in a casting,measured by sectioning the casting in a direction normal to the crystalgrowth direction, counting the number of primary arms over thecross-sectional area, and calculating an average spacing (typically byassuming a square array). Secondary dendrite arm spacing can be measuredby averaging the spacing between adjacent secondary dendrite armsobserved in a section taken parallel to the crystal growth direction.Dendrites that form during the solidification of SX and DS castings canbe distinguished from the surrounding material by differences inconcentration of certain alloy constituents.

Mechanical properties of DS and SX articles depend in part on theavoidance of high-angle grain boundaries, equiaxed grains, and defectsresulting from chemical or elemental interdendritic segregation duringthe directional solidification process. As an example, depending on theparticular chemistry of the superalloy, interdendritic segregation canresult in inhomogeneities such as embedded particles and elementalmicroconstituents of the alloy chemistry that accumulate ininterdendritic regions and tend to reduce the strength of the casting.The size of the embedded particles and pools of microconstituents can besignificantly reduced by a reduction in primary dendrite arm spacing inthe cast article. Interdendritic segregation can also result in theformation of surface freckles, which form during solidification aschains of very small equiaxed grains. Freckles can reduce fatigue lifeand act as grain initiators during the solidification process that causeunacceptable off-axial grains. Traditional approaches used to minimizethe presence or effect of dendritic segregation have includedpost-casting treatments, such as solid state diffusion heat treatmentsor mechanical working. However, these techniques are not feasible foraddressing dendritic segregation in gas turbine components and othercastings that are very large or formed with complex compositions.

The tendency for freckling has been shown to be dependent oncomposition, an example being the level of tantalum and/or carbon in analloy. Consequently, freckling has been addressed through carefulcontrol or modifications of superalloy compositions, as reported incommonly-assigned U.S. Pat. Nos. 5,151,249, 6,091,141 and 6,909,988.Recently, casting process parameters such as withdraw rate, coolingspeed, and the solid-liquid interface position have also been shown tohave an effect on freckle formation. Commonly-assigned U.S. Pat. No.6,217,286 to Huang et al. discloses a high-gradient casting process thatreduces freckling in castings having lengths of up to forty inches(about one hundred centimeters). Huang et al. teach that a high thermalgradient at the solidification front can be achieved with a baffleplaced between the mold and cooling zone, for example, a liquid bath orimpingement with an inert gas, to achieve a sufficiently uniform primarydendrite arm spacing and reduce freckling.

Huang et al. and other prior efforts to reduce freckling and othersolidification-related defects have been limited to castings that do notexceed lengths of forty inches (about one hundred centimeters), due inlarge part to size and weight limitations imposed by mold strength,furnace size, etc. Freckling and size/weight complications associatedwith directional solidification have essentially prevented theproduction and use of single-crystal and directionally-solidifiedcastings of sufficient size for certain applications, including thelast-stage buckets of land-based gas turbines. An example is thelast-stage buckets of the H and FB class gas turbines used in thepower-generating industry and manufactured by the assignee of thepresent invention. The lengths (about 30 inches (about 75 cm) or more),cross-sections and weights of these buckets have rendered them verydifficult to produce as SX and DS castings, particularly with respect toachieving microstructures that can be heat treated to obtain desiredmechanical properties. Consequently, last-stage buckets of the H and FBclass gas turbines have been limited to being produced as equiaxedcastings. However, the ability to produce these buckets as defect-freeSX and DS castings would achieve significantly improved mechanicalproperties, such as creep and low-cycle fatigue (LCF), and wouldtherefore be of great benefit to the overall performance and efficiencyof a large gas turbine.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides a process capable of producing metalliccastings having unidirectional crystal structures and lengths of onehundred centimeters or more, yet are substantially free of freckledefects. The present invention also provides metallic castings withunidirectional crystal structures that are formed by such a process.

According to a first aspect of the invention, the process includespouring a molten metal alloy into a cavity of a preheated mold within aheating zone, withdrawing the mold from the heated zone through a heatshield and into a cooling zone to directionally solidify the moltenmetal alloy, and then cooling the mold to produce the casting and theunidirectional crystal structure thereof that is substantially free offreckle defects having a size greater than one hundred centimeters.According to preferred aspects of the invention, the heat shieldoperates as a barrier to thermal radiation between the heated zone andthe cooling zone, and the mold is withdrawn at a rate that, incombination with the heat shield, maintains a thermal gradient of atleast 35° C./cm, for example, 50° C./cm or more, to solidify the moltenmetal alloy and form primary dendrite arms having an average spacingtherebetween of about 150 micrometers to about 500 micrometers. A highcooling rate of at least about 20° C./minute also appears to be a factorin achieving the desired primary dendrite arm spacing.

The unidirectional crystal structure of the casting can be a columnarsingle crystal microstructure (SX) with a preferred single crystaldirection of <001>, though crystalline structures having orientationsother than <001> are also within the scope of the invention, as arecolumnar polycrystalline microstructures (DS). Castings that can beproduced in accordance with the invention are well suited for componentsof a gas turbine, such as buckets, nozzles, and other components of gasturbines, and may be formed of nickel-base alloys and intermetallics,for example, a nickel aluminide (NiAl) intermetallic.

A significant advantage of this invention is that castings produced bythe process of this invention can far exceed in length, cross-section,and/or weight what was possible with prior casting techniques. Inparticular, heat-treatable, freckle-free last stage buckets ofland-based gas turbines can be produced to have single-crystal anddirectionally-solidified microstructures by this process, whereaslast-stage buckets and other castings with lengths exceeding one hundredcentimeters (and correspondingly large cross-sections and weights) werepreviously not possible. As such, the reduction of the incidence offreckling is greater than was expected for very large SX and DScastings, and the result is the absence of freckling that would beotherwise expected in SX and DS castings of these lengths,cross-sections and weights if produced under conventional processingconditions.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is representative of a turbine bucket that can be formed as asingle-crystal casting in accordance with an embodiment of the presentinvention.

FIGS. 2 and 3 represent cross-sectional views showing two steps of acasting operation to produce a large single-crystal turbine blade inaccordance with an embodiment of this invention.

FIG. 4 is a plot of primary dendrite arm spacing versus casting lengthfor castings produced in accordance with U.S. Pat. No. 6,217,286.

FIG. 5 is a plot of primary dendrite arm spacing versus casting lengthfor castings produced in accordance with embodiments of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides for the capability of producingunidirectionally-solidified castings beyond the capabilities andexpectations of prior directional casting technologies. The inventionbuilds upon a discovery disclosed in commonly-assigned U.S. Pat. No.6,217,286 that certain solidification process conditions, as evidencedby dendrite arm spacing, are capable of preventing freckles in castingsof up to forty inches (about one hundred centimeters) in length. Thecapability provided by the present invention was unexpected from U.S.Pat. No. 6,217,286, as evident from the teachings of this patent.Particularly unexpected was the ability to achieve and maintain asufficient thermal gradient capable of forming primary dendrite armswith an acceptable arm spacing that would result in the prevention offreckling. Because the thermal gradient is dependent on the cooling rateand solidification rate (approximately equal to the withdrawal rate) ofthe casting process (a simplified statement of this relationship is thatthermal gradient is approximately equal to the cooling rate divided bythe withdrawal rate), the invention requires close control over thecooling rate and withdrawal rate of the casting process.

The present invention reduces the tendency for freckling while achievingmechanical properties for which SX and DS castings are desired,particularly high temperature strength (including creep resistance) andfatigue properties for use in such applications as buckets (blades),nozzles (vanes), and other large components in the hot gas flow path ofa gas turbine. Of particular interest are the very large castingsrequired for last-stage turbine buckets used in land-based gas turbines,whose lengths (exceeding one hundred centimeters), cross-sections,and/or weights have prevented their manufacture as SX and DS castings.As an example, FIG. 1 represents a third-stage bucket 10 for aland-based gas turbine, such as gas turbines used in thepower-generating industry. The bucket 10 has an airfoil 12 and shank 14,with a dovetail 16 formed on the shank 14 for anchoring the bucket 10 toa turbine disk (not shown). Depending on the particular application, thelength of the bucket 10, from tip shroud 18 to dovetail 16, may be about30 inches (about 75 cm) or more, including lengths of about 50 inches(about 125 cm) or more. Furthermore, the weight (mass) of the bucket 10often exceeds 40 pounds (about 18 kg) and may exceed 50 pounds (about 23kg), with weights (mass) of even 100 pounds (about 45 kg) or more alsobeing possible. While the following discussion will focus on gas turbinebuckets, the invention also applies to a variety of other components,including components within the combustion section of a gas turbine.

Buckets of the type represented in FIG. 1 are conventionally produced asequiaxed castings of precipitation-hardened nickel-base superalloys,such as IN738, René 77, or Udimet® 500. As is in the case of first andsecond-stage turbine buckets, the performance of the bucket 10 could besignificantly improved if it could be unidirectionally cast to have acolumnar single crystal (SX) or columnar polycrystalline (DS)microstructure. The advantages of this invention will be described withreference to the bucket 10 of FIG. 1, though the teachings of thisinvention are generally applicable to other large components that couldbenefit from being unidirectionally cast.

The bucket 10 would also benefit from being cast from more advancedhigh-temperature materials. Of particular interest are superalloysspecifically formulated for casting as SX and DS castings, andintermetallics such as nickel aluminide (NiAl) intermetallic materials.Particular nonlimiting examples of superalloys that could be usedinclude René N5 (nominal composition, by weight, about 7.5% Co, 7.0% Cr,6.5% Ta, 6.2% Al, 5.0% W, 3.0% Re, 1.5% Mo, 0.15% Hf, 0.05% C, 0.004% B,0.01% Y, the balance nickel), René N4 (nominal composition, by weight,about 9.75% Cr, 7.5% Co, 4.2% Al, 3.5% Ti, 1.5% Mo, 6.0% W, 4.8% Ta,0.5% Nb, 0.15% Hf, 0.05% C, 0.004% B, the balance nickel), GTD111(nominal composition, by weight, about 14.0% Cr, 9.5% Co, 3.0% Al, 4.9%Ti, 1.5% Mo, 3.8% W, 2.8% Ta, 0.010% C, the balance nickel), and GTD444(nominal composition, by weight, about 9.75% Cr, 7.5 Co, 3.5% Ti, 4.2%Al, 6% W, 1.5% Mo, 4.8% Ta, 0.08% C, 0.009% Zr, 0.009% B, the balancenickel).

As known in the art, freckles form in part as a result of molten metalconvection in the casting mold which disrupts the unidirectionalsolidification process, producing irregularities seen on SX and DScasting surfaces as little chains of equiaxed crystals. Furthermore,freckles can act as grain initiators during the solidification processthat cause unacceptable off-axial grains, and may reduce fatigue life ofthe casting. According to one aspect of the invention, external andinternal freckling can be inhibited and even eliminated in SX and DScastings exceeding one hundred centimeters in length (withcorrespondingly large cross-sections and weights) by achieving greatercontrol of the primary dendrite arm spacing at these casting lengths,more particularly achieving finer dendrite arm spacing at these castingsizes to reduce buoyancy impact, which in turn can be attained byimproving the thermal separation between the heating and cooling zonesof a unidirectional casting process to achieve an even greater thermalgradient (for example, 80° C./cm and higher) at the solidification frontof the casting. In particular, U.S. Pat. No. 6,217,286 utilized athermal gradient of up to about 80° C./cm to obtain primary dendrite armspacings of about 150 micrometers to less than 800 micrometers,preferably about 150 micrometers to about 650 micrometers, and morepreferably about 150 micrometers to about 350 micrometers for castingsof up to one hundred centimeters. These spacings roughly correspond toarm spacing-casting length ratios of up to about 8.0micrometers/centimeter for a 100-cm casting. In comparison, the presentinvention achieves freckle-free SX castings by narrowly limiting theprimary dendrite arm spacing to a range of about 150 micrometers toabout 500 micrometers, more preferably about 250 to about 450micrometers, and most preferably about 325 to 450 micrometers (about 13to about 18 mils) for castings exceeding one hundred centimeters,corresponding to a maximum arm spacing-casting length ratio of not morethan 5 micrometers/centimeter and preferably not more than 4.5micrometers/centimeter for a 100-cm casting. For comparison, FIG. 4plots the preferred and most preferred ranges from the U.S. Pat. No.6,217,286 patent as lines 50 and 48, respectively, over casting lengthsof four to forty inches (about ten to about one hundred centimeters),while FIG. 5 plots the ranges 52 and 54 for, respectively, the preferredand most preferred primary dendrite arm spacings of this invention forcastings with lengths that exceed forty inches (one hundred centimeters)For the sake of comparison, the line 50 for the preferred range of U.S.Pat. No. 6,217,286 is plotted beyond casting lengths of forty inches,though there was no expectation or suggestion in U.S. Pat. No. 6,217,286that castings longer than forty inches could be cast under theseconditions without producing unacceptably high levels of freckles.Furthermore, ceramic molds available to Huang et al. limited the size ofcastings that could have been considered by Huang et al.

In view of the above, an important aspect and unexpected result of thepresent invention is that large castings with lengths exceeding onehundred centimeters (with correspondingly large cross-sections andweights) can be manufactured having a fine dendrite arm spacing of notmore than 500 micrometers, for example, about 150 to about 500micrometers, to avoid freckling. More particularly, the primary dendritearm spacing is most preferably between 325 and 450 micrometers. Thetargeted spacing may be correlated to the length of the casting,corresponding to a spacing/length ratio of about 0.75 to about 5.0, ormore narrowly about 1.625 to about 4.5 micrometers per centimeter, andmore preferably about 2.25 to about 3.25 micrometers per centimeter.Other potential casting defects may also be minimized with thisinvention, including high angle boundaries that tend to form atprotruded sections of castings, and grains that form streaks (slivers)in the microstructure.

FIGS. 2 and 3 represent a shell mold 20 of a type suitable for producinga single-crystal casting of this invention. As known in the art, themold 20 is preferably formed of a material such as alumina or silica,and has an internal cavity 22 corresponding to the desired shape of acasting 32, represented as a turbine bucket. As such, the cavity 22 isconfigured to produce the casting 32 with an airfoil portion 34, shank36, and dovetail 38, and may contain cores (not shown) for the purposeof forming cooling passages within the casting 32. The mold 20 is shownsecured to a chill plate 24 and placed in a heating zone 26 (forexample, a Bridgman furnace) to heat the mold 20 to a temperature equalto or above the melting temperature of the alloy, and more particularlyabove the liquidus temperature of the alloy. A cooling zone 42 isrepresented as being located directly beneath the heating zone 26, and abaffle or heat shield 44 is represented as being between and separatingthe heating and cooling zones 26 and 42. The cooling zone 42 may be atank containing a liquid cooling bath 43, such as a molten metal, or aradiation cooling tank that may be evacuated or contain a gas at ambientor cooled temperature. The cooling zone 42 may also employ gasimpingement cooling (for example, see U.S. Pat. No. 7,017,646 to Ballielet al.) or a fluidized bed (for example, see U.S. Pat. No. 6,443,213).Particularly suitable liquids for the cooling bath 43 include molten tinat a temperature of about 235 to about 350° C. and molten aluminum at atemperature of up to about 700° C., with molten tin believed to beespecially suitable because of its low melting temperature and low vaporpressure.

The heat shield 44 is situated to be in close contact with the lower endof the heating zone 26 and the cooling zone 42, and in the case of acooling bath 43 may float on its surface. The purpose of the heat shield44 is to insulate the cooling zone 42 from the heating zone 26, andparticularly form a barrier to thermal radiation emitted by the heatingzone 26, thereby promoting a steep thermal gradient between the mold 20and the cooling bath 43. The heat shield 44 may be a single layer ormultiple layers of rigid and/or flexible thermal barrier materials, suchas flowing graphite raft, a refractory felt material, or a high meltingpoint metal. The heat shield 44 is configured to have a variable-sizedopening 45 that, as represented in FIG. 2, enables the heat shield 44 tofit closely around the shape of the mold 20 as it is withdrawn from theheating zone 26, through the heat shield 44, and into the liquid coolingbath 43.

The casting process is preferably carried out in a vacuum or an inertatmosphere, with the mold 20 preheated to a temperature above thealloy's liquidus temperature, as a nonlimiting example, about 1370° C.to about 1600° C. The molten alloy is poured into the preheated mold 20,after which unidirectional solidification is initiated by withdrawingthe base of the mold 20 and chill plate 24 downwardly at a fixedwithdrawal rate into the cooling zone 42, until the mold 20 is entirelywithin the cooling zone 42 as represented in FIG. 3. The temperature ofthe chill plate 24 is preferably maintained at or near the temperatureof the cooling zone 44, such that dendritic growth begins at the lowerend of the mold 20 and the solidification front travels upward throughthe mold 20. The casting 32 grows epitaxially (for example, with the<100> orientation) based on the crystalline structure and orientation ofa small block of single-crystal seed material 28 at the base of the mold20, from which a single crystal forms from a crystal selector 30, forexample, a pigtail sorting structure. The columnar single crystalbecomes larger in the enlarged section of the cavity 22. A bridge 40connects protruding sections of the casting 32 with lower sections ofthe casting 32 so that a unidirectional columnar single crystal formssubstantially throughout the casting 32. The casting 32 is deemed to bea substantially columnar single crystal if it does not have high anglegrain boundaries, for example, greater than about twenty degrees.

Uniform primary dendrite arm spacings are achieved by the strongunidirectional thermal gradients imposed on the casting 32 as a resultof the heat shield 44 and the cooling zone 42. According to a preferredaspect of the invention, the thermal gradient at the solidificationfront of the casting 32 is greater than 35° C./cm, preferably greaterthan 50° C./cm, and more preferably greater than 80° C./cm. Thermalgradients of less than 50° C./cm and particularly less than 30° C./cmare believed to be unacceptable for attaining the primary dendrite armspacing in large castings of primary interest to this invention. Basedon their mathematical relationship, the high thermal gradients of thisinvention also require high cooling rates relative to the withdrawalrate used, the latter of which can be up to at least twenty inches/hour(about 8.5 mm/minute).

Those skilled in the art will appreciate that a DS casting can beproduced in a similar manner, though with modifications to the mold 20,such a growth zone at the base of the mold 20 that is open to the chillplate 24, and omission of the seed material 28 and/or crystal selector30.

In experiments leading to the present invention, last-stage bucketssimilar to the representation of FIG. 1 were cast. The castings wereabout 30 to about 50 inches (about 750 to about 1250 millimeters) inlength. The compositions of the buckets were the nickel-basedsuperalloys René N4 and GTD444, both of which are specificallyformulated for SX and DS castings. Single-crystal castings were thenprepared in accordance with commercial practices for the alloys,generally in accordance with the casting process described above. Thecasting molds were about ten inches (about 25 cm) longer than theirrespective castings, and filled to contain up to about 400 lbs. (about180 kg) of molten alloy. The casting furnace temperature was about 2750°F. (about 1510° C.). Cooling was by a liquid bath of molten tinmaintained at a temperature of about 240° C., and the thermal gradientin the castings during cooling was about 85° C./cm. A conventionalwithdrawal rate of about three inches/hour (about 1.25 mm/minute) wasused to produce a casting as a baseline comparison, while other castingswere produced using higher experimental rates of about six to twelveinches/hour (about 2.5 to about 5 mm/minute). Based on these values, thecooling rates were about 10° C./minute when using the conventionalwithdrawal rate (three inches/hour), and about 20 to about 40° C./minuteusing the higher withdrawal rates (six and twelve inches/hour).

Following the casting operation, primary dendrite arm spacings in thecastings were measured by metallography, and evidence of freckling wasexamined by macro-etching the casting surfaces, followed bymetallographic examination. From the examinations, it was observed thatthe grain structure broke down in those casting produced at theconventional withdrawal rate (corresponding to a cooling rate of about10° C./minute). Furthermore, the casting had many freckles at itsthicker sections and at sections where the cross-section changed, suchas the tip shroud. Dendrite spacing was about 25 to about 30 mils (about635 to about 760 micrometers), which is within the broad range acceptedin Huang et al. Nonetheless, the casting was unable to be heat treateddue to excessive segregation in its microstructure, resulting inincipient melting.

In contrast, the experimental castings produced at the higher withdrawalrates (corresponding to cooling rates of about 20 to about 40°C./minute) did not contain any freckles. Furthermore, the grains werestraight, evidencing that grain growth was not influenced by other heatextraction directions. Dendrite spacing was about 16 to about 21 mils(about 400 to about 530 micrometers), corresponding to a minimumdendrite spacing to casting length ratio of about 0.32 μm/cm. Finally,and significantly, their microstructures allowed the castings to be heattreated to obtain desired mechanical properties for the buckets.

Based on these results, it was concluded that SX and DS castings formedof superalloys formulated for SX and DS and cast to sizes produced inthe experiment (lengths of about 760 mm (about 30 inches) and longer,weights exceeding 40 pounds (about 18 kg), and correspondingly largecross-sections benefit from relatively high thermal gradients(preferably greater than 80° C./cm and more preferably about 85° C./cmor more). The benefit of high thermal gradients appears to also dependon the use of relatively high cooling rates (greater than 10° C./minute,such as about 20° C./minute or more) relative to withdrawal rate. Theprocess may also employ relatively high withdrawal rates, for example,greater than 1.25 mm/minute, such as about 2.5 to about 5 mm/minute,though lower and higher withdrawal rates are also within the scope ofthis invention.

While the invention has been described in terms of specific embodiments,it is apparent that other forms could be adopted by one skilled in theart. For example, the physical configuration of the castings coulddiffer from that shown, and materials and processes other than thosenoted could be used. Therefore, the scope of the invention is to belimited only by the following claims.

1. A process of producing a metallic casting having a length greaterthan one hundred centimeters and a unidirectional crystal structure thatis substantially free of freckle defects, the process comprising:pouring a molten metal alloy into a cavity in a preheated mold locatedwithin a heating zone, the cavity having the shape of the casting;withdrawing the mold from the heating zone, through a heat shield, andinto a cooling zone to directionally solidify the molten metal alloy,the heat shield operating as a barrier to thermal radiation between theheating zone and the cooling zone, the mold being withdrawn at a ratethat, in combination with the heat shield, maintains a thermal gradientof greater than 50° C./cm in the molten metal alloy to solidify themolten metal alloy and form primary dendrite arms having an averagespacing therebetween of about 150 micrometers to about 500 micrometers;and then cooling the mold to produce the casting and the unidirectionalcrystal structure thereof that is substantially free of freckle defectshaving a size greater than one hundred centimeters.
 2. The processaccording to claim 1, wherein the casting has a mass of at least about18 kg.
 3. The process according to claim 1, wherein the thermal gradientis greater than 80° C./cm.
 4. The process according to claim 1, whereinthe withdrawal rate is greater than 1.25 mm/minute.
 5. The processaccording to claim 1, wherein in combination the thermal gradient andthe withdrawal rate result in a cooling rate of at least 20° C./minute.6. The process according to claim 1, wherein the average spacing betweenthe primary dendrite arms is about 325 micrometers up to about 450micrometers.
 7. The process according to claim 1, wherein the casting ischaracterized by a ratio of the average spacing of the primary dendritearms to the length of the casting of about 0.75 to about 5.0 micrometersper centimeter.
 8. The process according to claim 1, wherein theunidirectional crystal structure has a columnar single crystalmicrostructure.
 9. The process according to claim 1, wherein theunidirectional crystal structure has a columnar polycrystallinemicrostructure.
 10. The process according to claim 1, wherein the metalalloy is chosen from the group consisting of nickel-base superalloys andintermetallic alloys.
 11. The process according to claim 1, wherein thecasting is a component for a gas turbine.
 12. The process according toclaim 11, wherein the component is a last-stage bucket of a land-basedgas turbine.
 13. The casting produced according to the process ofclaim
 1. 14. The casting according to claim 13, wherein theunidirectional crystal structure has a columnar single crystalmicrostructure.
 15. The casting according to claim 13, wherein theunidirectional crystal structure has a columnar polycrystallinemicrostructure.
 16. The casting according to claim 13, wherein the metalalloy is chosen from the group consisting of nickel-base superalloys andintermetallic alloys.
 17. The casting according to claim 13, wherein theaverage spacing between the primary dendrite arms is about 325micrometers to about 450 micrometers.
 18. The casting according to claim13, wherein the casting is characterized by a ratio of the averagespacing of the primary dendrite arms to the length of the casting ofabout 2.25 to about 3.25 micrometers per centimeter.
 19. The castingaccording to claim 13, wherein the casting is a component for a gasturbine with a mass of at least about 18 kg.
 20. The casting accordingto claim 19, wherein the component is a last-stage bucket of aland-based gas turbine.